Opposite Field Hitting
Placement hitting is all about stance, alignment, step and stroke.
You don't want to give away where you want to go with the ball, so I try to keep a comfortable, consistent stance.
That usually means feet about shoulder width, hips and shoulders aligned and pointed at second base, hands back and high. High hands is important in slow / high arc softball; if you drop your hands you'll tend to upper cut the ball which results in more pop ups.
You'll find that when you keep your hands high in your stance, it will level your swing and lets you hit more line drives.
Alignment:
To go opposite field I stand further from the plate in the batters box and closer to the pitcher. You're really looking for an outside pitch to take to the opposite field and by standing away in the batters box it's harder for the pitcher to jam you and still throw strikes.
(Note: a good pitcher who sees you off the plate may deliberately try to pitch you outside so don't stand so far that you can't get to the outside of the plate or you may get struck out.)
Standing closer to the pitcher means that a strike is going to descend deeper back in your swing - a deeper pitch is easier to take to the opposite field. If the pitcher knows you want to go that way he may deliberately try to pitch you short.
Step:
To go opposite field, take your step that way. I never move my back foot unless I have to, I prefer to let my front foot close my stance on the step, basically I step towards the pitcher and towards the plate - placing my heel slightly past the line where my toes were. That closes your stance, turns your hips towards the right and helps you take the ball that way.
Swing:
Be patient. You need to wait on the ball to go opposite field. Keep the hands high because it's easy to uppercut when you're trying to go that way. You want to get your hands out in front of the barrel of the bat on your swing.